Optical fibers provide significantly higher data rates than electronic paths. However, effective utilization of the greater bandwidth inherent in optical signal paths requires optical cross-connect switches. In a typical telecommunications environment, the switching of signals between optical fibers utilizes an electrical cross-connect switch. The optical signals are first converted to electrical signals. After the electrical signals have been switched, the signals are again converted back to optical signals that are transmitted via the optical fibers. To achieve high throughput, the electrical cross-connect switches utilize highly parallel, and highly costly, switching arrangements. However, even with such parallel architectures, the cross-connect switches remain a bottleneck.
A number of optical cross-connect switches have been proposed; however, none of these have successfully filled the need for an inexpensive, reliable, optical cross-connect switch. One class of optical cross-connects depends on wavelength division multiplexing (WDM) to affect the switching. However, this type of system requires the optical signals being switched to have different wavelengths. In systems where the light signals are all at the same wavelength, this type of system requires the signals to be converted to the desired wavelength, switched, and then be re-converted to the original wavelength. This conversion process complicates the system and increases the cost.
A second type of optical cross-connect utilizes total internal reflection (TIR) switching elements. A TIR element consists of a waveguide with a switchable boundary. Light strikes the boundary at an angle. In the first state, the boundary separates two regions having substantially different indices of refraction. In this state the light is reflected off of the boundary and thus changes direction. In the second state, the two regions separated by the boundary have the same index of refraction and the light continues in a straight line through the boundary. The magnitude of the change of direction depends on the difference in the index of refraction of the two regions. To obtain a large change in direction, the region behind the boundary must be switchable between an index of refraction equal to that of the waveguide and an index of refraction that differs markedly from that of the waveguide.
One class of prior art TIR elements that provide a large change in index of refraction operates by mechanically changing the material behind the boundary. For example, U.S. Pat. No. 5,204,921, Kanai, et al. describes an optical cross-connect based on an array of crosspoints in a waveguide. A groove at each crosspoint, may be switched "on" or "off," depending upon whether the groove is filled with an index-matching oil. The index-matching oil has a refractive index close to that of the waveguides. An optical signal transmitted through a waveguide is transmitted through the crosspoint when the groove is filled with the matching oil, but the signal changes its direction at the crosspoint through total internal reflection when the groove is empty. To change the cross-point switching arrangement, grooves must be filled or emptied. In the system taught in this patent, a "robot" fills and empties the grooves. This type of switch is too slow for many applications of interest.
A faster version of this type of TIR element is taught in U.S. Pat. No. 5,699,462 which is hereby incorporated by reference. The TIR taught in this patent utilizes thermal activation to displace liquid from a gap at the intersection of a first optical waveguide and a second optical waveguide. In this type of TIR, a trench is cut through a waveguide. The trench is filled with an index-matching liquid. A bubble is generated at the cross-point by heating the index matching liquid with a localized heater. The bubble must be removed from the crosspoint to switch the cross-point from the reflecting to the transmitting state and thus change the direction of the output optical signal.
If the bubble contains noncondensable gases (such as air), it takes too long (minutes) to collapse when the heater is turned off. This is not acceptable for most applications which require a faster cycle time. Such a gas bubble can be removed by applying a force to the bubble to move it out of the optical path, to one side.
The bubble can also be moved to another section of the trench by increasing the pressure on one side of the bubble. Such pressure increases can be accomplished by heating the fluid on one side of the cross-point or by physically displacing the fluid on one side of the cross-point so as to push or pull the bubble away from the cross-point. If the walls of the trench are parallel to one another, the displacement must be sufficient to move the entire bubble out of the cross-point area. Such large displacements require relatively long times or expensive hardware.
Broadly, it is the object of the present invention to provide an improved cross-point for use in cross-connect switches and the like.
It is a further object of the present invention to provide a cross-point in which the bubble clearing time is shorter than in prior art cross-point switches.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.